This invention relates to voltage controlled oscillators (VCOs) and, more particularly, to a novel circuit topology that permits VCOs to operate down to very low supply voltages.
Due to the emergence of the mobile telecommunications market, the need for small, inexpensive, and low-power RF circuit components is paramount. By integrating more and more functions on the same die, single-chip transceivers are only now becoming a reality.
One of the major challenges in the design of an inexpensive transceiver system is frequency synthesis of the local oscillator signal. Frequency synthesis is usually done using a phase-locked loop (PLL). A PLL typically contains a phase detector, a filter and a voltage-controlled oscillator (VCO). The feedback action of the loop causes the output frequency to be some multiple of a supplied reference frequency. This reference frequency is generated by the VCO whose output frequency is variable over some range according to an input control voltage.
Despite numerous advances in the art, VCOs still remain as one of the most critical design components in RF transceivers. The most important parameters of a VCO are phase noise, power consumption and frequency tuning range. Specifically, it is of major importance to build low-power, low-phase-noise oscillators. This has only been possible with oscillators based on the resonance frequency of an inductor-capacitor (LC) tank circuit.
Traditionally, RF designers have always used external resonator elements for this tank circuit to achieve good phase noise performance. However, higher degrees of integration are desirable because they lead to lower manufacturing costs due to reduced board complexity and improved reliability. While offering the advantages of low cost and reduced sensitivity to packaging parasitics, VCOs comprising integrated LC resonators suffer from the low quality-factor (Q) of the inductor used in the LC tank circuit. This, in turn, leads to poorer noise performance for a given level of power consumption.
On-chip voltage controlled oscillators (VCOs), therefore, continue to be a subject of intense research as designers struggle to meet phase noise requirements using low Q-factor on-chip inductors. However, as technologies progress and on-chip inductor Qs continue to rise due to thicker metalisation and especially with the introduction of copper (Cu) interconnects into some advanced processes, their performance will continue to improve. Higher Q inductors lead to lower loss oscillators because a higher Q-factor means a higher parallel resistance of the tank at resonance. This implies a larger oscillation amplitude and a reduced output noise spectral density, for the same current consumption. Undoubtedly, then, on-chip VCOs will become more widely used in receiver integrated circuits (ICs), resulting in low-cost highly integrated ICs.
A perfectly lossless resonant circuit is very nearly an oscillator, but lossless elements are difficult to realize. Overcoming the energy loss implied by the finite Q of practical resonators with the energy supplying action of active elements is one potentially attractive way to build practical oscillators. The basic ingredients in all LC feedback oscillators, then, are simple: one transistor plus a resonator. In theory, there is no limit to the number of ways to combine a resonator with a transistor or two to make an oscillator.
A number of designers have used the differential Colpitts common-collector and common-base VCO configurations with reasonably good results. These configurations usually provide good output power, are insensitive to parasitics and have relatively good phase noise performance. However, compared to the simpler negative transconductance (xe2x88x92Gm) oscillator that has been gaining popularity over the last few years, they call for a complicated circuit with complicated biasing requirements.
In order to guarantee oscillation, the net resistance across the LC tank of an oscillator must be negative. This negative resistance is used to offset the positive resistance of all practical resonators, thereby overcoming the damping losses and reinforcing the oscillation. The negative transconductance (xe2x88x92Gm) oscillator uses a cross-coupled differential pair to synthesize the negative resistance.
The basic topology for the xe2x88x92Gm oscillator is shown in FIG. 1. The oscillator may be viewed as consisting of three parts: an LC resonant tank comprising an inductor L and varactors Cvar, a negative resistance generation (positive feedback) network comprising bipolar npn transistors Q1 and Q2 and a biasing network comprising a current source Ibias. Bipolar npn transistors Q1 and Q2 form a negative resistance generator in parallel with the LC tank that sets the frequency of oscillation. From the point of view of the LC tank the active circuitry (i.e. transistors Q1 and Q2) cancels the losses due to the finite Q of the LC resonator tank Varactors Cvar are used in place of fixed capacitors to provide a tuning scheme using a control voltage Vcont applied via a resistor Rcont. The supply voltage Vcc is fed into the circuit through the center tap of a symmetric differential inductor L. A single inductor is usually preferred in this implementation because of the superior quality factor Q and reduced chip area compared with using two inductors connected in series. Finally, in order to maintain a constant total current shared by the two transistors, a large emitter resistance must be realized. A constant current source Ibias is typically used to simulate this high resistance because the large voltage drop associated with large resistors makes them very impractical for this application.
The current source Ibias aids in controlling the swing of the oscillator and may be implemented with any of the common current generator circuits already in existence. It is apparent, then, that the circuit of FIG. 1 will contain a minimum of two transistors xe2x80x98stackedxe2x80x99 on top of one another (one transistor from the differential pair and one transistor from the current source). Accordingly, the VCO of FIG. 1 will require a minimum supply voltage Vcc equal to twice the turn-on voltage of a typical transistor. It is this stacking of transistors, therefore, which limits the minimum supply voltage at which a VCO may operate.
Recent trends have seen the desire for lower and lower supply voltages in radio frequency (RF) components as this leads to lower power consumption and, therefore, longer battery life. As well, lower voltages and less current means that mobile products can be made to require fewer battery cells leading to lighter, more compact devices. To achieve good phase noise from VCOs at low supply voltages is, however, among the toughest challenges facing the RF designer. This is due to the fact that the phase noise of the VCO is dependent on the output power and, therefore, on the voltage swing of the oscillator. Therefore, as the supply voltage drops, then so does the available voltage swing of the oscillator. Oscillator designers, therefore, do not generally like to lower the supply voltages of their circuits because of this negative impact on performance. They will, therefore, do so only as a last resort.
Furthermore, VCOs are not generally well understood in the design community. As a result, designers are often hesitant to try new topologies primarily because they are unsure of how they will perform. As such, most designers have a small number of xe2x80x98favoritexe2x80x99 oscillator circuits that they adapt to meet changing performance requirements.
The present invention discloses a novel topology for a low-voltage voltage-controlled oscillator (VCO). In particular, the invention is based on the topology of a negative transconductance oscillator due to its intrinsically simple biasing scheme.
According to a broad aspect of the invention, a voltage-controlled oscillator circuit for use with a low-voltage power supply is provided. The oscillator circuit has a parallel resonant network connected to a first power rail and a pair of controllable current sources connected to the resonant network to form a negative resistance generation network. Each controllable current source of the pair has a first and second conducting terminal and a control terminal. The pair of controllable current sources is arranged so that the first conducting terminal of the first controllable current source is AC coupled to the control terminal of the second controllable current source. Similarly, the first conducting terminal of the second controllable current source is AC coupled to the control terminal of the first controllable current source. The second conducting terminal of each controllable current source is connected to a second power rail, preferably ground. Finally, a biasing network connected between the first and second power rails is coupled to the control terminal of each controllable current source of the negative resistance generation network to bias the controllable current sources with DC current.
The topology of the present invention eliminates transistor stacking and, in so doing, allows the VCO to operate down to very low supply voltages. Low voltage operation is very desirable for wireless circuits because they are often put into products that operate from a battery power supply. Low-voltage operation translates into lower power consumption and, therefore, longer battery life. Advantageously, mobile products can then be made to require fewer battery cells leading to lighter, more compact devices.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings.